Such arrangements are used for measuring purposes and find their application in such devices as multiphase electricity meters for the exact conversion of a multiphase output into a frequency which is proportional to the sum of outputs.
A multiphase signal u.sub.H,1 ; u.sub.H,2 ; u.sub.H,3 comprises either measuring signals, e.g. electric voltages or currents, or signals which are derived from measuring signals such as electrical outputs which are known to be equal to the product of a currant and a voltage.
The structure and functioning of an arrangement of the kind mentioned above for a single-phase signal is known from U.S. Pat. No. 41 24 821. In that patent the principle of periodic polarity reversal is used for the elimination of an offset voltage which as a rule affects the precision of measurement as a linear error in the form of a part of an input signal which cannot be compensated and is time or temperature dependent. Here the signal to be converted as well as the direction of transmission of the arrangement are polarity reversed periodically and simultaneously so that the two polarity sign changes of the signal to be converted cancel each other out. The influence of the offset voltage on the other hand, which is subjected only to the polarity sign change of the transmission direction, is alternately added to or subtracted from the signal during each half-period of a switch-over signal so that the integration of the offset voltage over a whole number of periods of the switch-over signal results in the value zero, on condition that both half periods of the switch-over signal are of exactly the same length. Since a residual charge which is other than zero is still present as a rule in an integration capacitor at the moment of polarity reversal, the residual charge becomes effective after the polarity reversal by causing an erroneous polarity and causing the wrong measuring result to be obtained. Considered over a longer measuring period, this error need not exert a negative influence on the median value of the impulse frequency, but it leads to a momentary variation in any case, i.e. to a modulation of the impulse frequency. To avoid this, the state of the art provides for the synchronization of the polarity reversal with the output signal of a comparator which monitors the capacitor voltage so that the polarity reversal always occurs at the crossover of the capacitor voltage, i.e. at the instant when the residual charge of the capacitor is zero. However this solution causes the two half-periods of the switch-over signal to not always be exactly equal but, on a statistical average, to be of equal duration and thus to lead again to an irregular modulation of the output frequency of the arrangement which complicates the latter's calibration which is possible only after an unduly long period of time.
It is the object of the instant invention to improve the known arrangement in such manner that it can be used for a multiphase signal in a manner which avoids the modulation of the output frequency of the arrangement by unequal half-periods of the switch-over signal as well as modulation by residual capacitor charges which are still present at the instant of polarity reversal so that rapid and exact calibration of the equipment, without long waiting periods is thus made possible.